The present invention relates to golf ball materials which have a good thermal stability, flow and processability and from which there can be obtained high-performance golf balls having excellent properties such as rebound resilience, durability and elasticity. The invention also relates to methods for preparing such golf ball materials, and to golf balls which include as a component therein a molded part made from such a golf ball material.
In recent years, ionomeric resins have been widely used as cover materials for golf balls. Ionomeric resins are ionic copolymers of an olefin such as ethylene with an unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic acid, in which some of the acid groups are neutralized with metal ions such as sodium, lithium, zinc or magnesium. These resins provide excellent characteristics in terms of durability, rebound resilience and scuff resistance of the ball.
At present, the base resins used in golf ball cover materials are generally ionomeric resins, but various improvements are being made to cope with the constant desire by players for golf balls having a high rebound resilience and an excellent flight performance.
For example, to improve the rebound resilience and the cost characteristics of ionomer cover materials, U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760 and International Application WO 98/46671 describe cover materials composed of an ionomeric resin to which a large amount of a metallic soap has been added.
However, the metallic soap in these cover materials undergo decomposition and vaporization during injection-molding, generating a large amount of fatty acid gases. As a result, molding defects tend to arise. Moreover, the gases that have formed deposit on the surface of the molded part, markedly lowering its paintability. In addition, depending on the type of metallic soap used, significant declines in processability and rebound resilience sometimes occur, making the cover material entirely unfit for practical use.
In ionomer cover materials, it is a common practice to blend together ionomers containing different metals so as to improve the rebound resilience and durability (e.g., scuff resistance and low-temperature impact resistance), and to use the resulting blend to form golf ball covers. For example, U.S. Pat. No. 3,819,768 describes the mixture of a sodium ionomer of an ethylene-(meth)acrylic acid copolymer with a zinc ionomer of an ethylene-(meth)acrylic acid copolymer to form a golf ball cover material having rebound resilience improved, and the use of the resulting blend in golf ball covers. However, since this is a two-step process in which, first, ionomers of the different metals are prepared, then the respective ionomers are melt-blended together to give the golf cover material, there have been concerns over deterioration in properties due to thermal history through the melt-blending and incomplete blending of both ionomers as well as concerns over increased costs.
An ionomer that has recently been developed for use as a golf ball material is a homogeneous-phase, high-rebound resilience material having an interpenetrating polymer network (IPN) structure (U.S. Published Patent Application No. 2004/0044136). The ionomer is prepared by blending a first component such as an ethylene-(meth)acrylic acid copolymer with a different type of thermoplastic resin as a second component to form a resin composition, then adding as a third component a metal ionic species to neutralize the acid groups in the first component dispersed in the resin composition. However, since the resin material prepared by this method contains only one type of metal ion, there is a concern that the physical properties are inferior to those of resin materials containing a combination of different metal ions. Moreover, since a solid (i.e., a powder or granular material) such as a metal oxide, metal hydroxide or metal carbonate as the metal ionic species is used as it is, and also in case of a high acid content in the first component, the addition of a large amount of the metal ionic species is required for neutralizing the acid groups, during mixing the metal ionic species with the resin components, there are concerns about both poor dispersion of the solid metal ionic species in the resin components and leaving some of the metal ionic species unreacted. In addition, given that a partial neutralization reaction (incomplete degree of neutralization) occurs and that the target degree of neutralization cannot be achieved in a one-step reaction through one-pass extrusion, more than one-pass extrusion be done, which concerns about lowering the physical properties of the resulting ionomer composition.